1. Field of the Invention
The present invention relates to a technique for recording an image onto a printing plate and a technique for modifying width of an image.
2. Description of the Background Art
Conventionally, a technique called CTP (Computer To Plate) for making a printing plate by directly outputting data of an image from a computer to a printing plate has been used. This CTP technique remarkably reduces process for plate making. A printing apparatus equipped with a plate-making apparatus has been developed as disclosed in Japanese Patent Application Laid Open Gazette Nos. 2000-280439 and 2002-67268. Such an apparatus allows quick transition from a plate-making process to a printing process, thereby achieving various types of printings with high efficiency.
Further, to achieve high-speed plate making in the CTP technique used are a multibeam irradiation where a plurality of light beams are emitted to a printing plate and a “spiral exposure (spiral image recording)” where a plurality of light beams are emitted while a writing head is continuously moved along a direction of rotation axis of a drum wound with a-printing plate, as disclosed in Japanese Patent Application Laid Open Gazette No. 2000-280439.
In a highly-developed printing apparatus for commercial use, there arises a micro-level misregistration of printing position, of about several hundred micrometers, which is caused by errors in machining and setting of parts, extension and contraction (caused by humidity) in printing which varies among papers (pieces of paper), or the like. Particularly, a paper extends mainly in a printing direction due to dampening water and also extends in a direction orthogonal to the printing direction at the end of paper in the printing direction. Such a phenomenon as extension of paper is called “fan-out”. The misregistration of printing position, which may become a cause of failure in precise registration of images obtained by printing with various colors in multicolor printing, is not negligible.
In order to prevent the misregistration due to fan-out, conventionally has been used a method where a printing plate is distorted by a skilled workman through technical application of force in setting the printing plate around a plate cylinder. But this method depends on the ability of the workman, so is not preferable one.
On the other hand, as disclosed in Japanese Patent Application Laid Open Gazette No. 2002-67268, it is suggested to change movement of irradiation positions of light beams in plate making by hardware. In this case, change in a main scan direction (direction of rotating a drum which holds the printing plate) is made by shifting a writing clock (for example, by changing a frequency multiplication factor used for obtaining the writing clock) and change in a subscan direction is made by changing a reducing projection ratio of light beam while moving a writing head towards a direction opposite to the subscan direction in a non-writing region near a plate gripper, i.e., performing “switchback”.
The method where the irradiation positions of light beams and irradiation timing are changed by hardware is good in that change of less than 1 dot (e.g., 1/10 dot) is possible and it is not necessary to modify image data, but the mass of a writing head is very great and for switchback, a large force is needed. As a result, it is not possible to efficiently utilize the characteristic feature of spiral exposure that allows stable movement of the light irradiation positions and it becomes difficult to perform a quick control of irradiation positions by micrometers.
Since the fan-out of paper is not linear in the printing direction, a complicate control is required to perform modification of image only by writing control in the main scan direction and subscan direction.